Drilling lost circulation material

ABSTRACT

An engineered composition for reducing lost circulation in a well includes a mixture of coarse, medium and optional fine particles, and a blend of long fibers and short fibers. The long fibers are rigid and the short fibers are flexible. The long fibers form a tridimensional mat or net in the lost-circulation pathway that traps the mixture of particles and short flexible fibers to form a mudcake. The mixture of particles and blend of fibers may be added to water-based and oil-based drilling fluids. The composition, size, and concentration of each component of the mixture of particles and blend of fibers may be fine-tuned for each application.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Drilling fluids have a number of functions, including but not limitedto, lubricating the drilling tool and drill pipe which carries the tool,providing a medium for removing formation cuttings from the well to thesurface, counterbalancing formation pressure to prevent the inflow tothe wellbore of gas, oil, and/or water from permeable or porousformations which may be encountered at various levels as drillingprogresses, preventing the loss of drilling fluids to void spaces and topermeable or porous formations, maintaining hole stability prior tosetting the casing, minimizing formation damage, and holding the drillcuttings in suspension, especially in the event of a shutdown indrilling and interruption of pumping of the drilling mud.

Drilling fluid additives in time can form a thin, low permeabilityfilter cake (mud cake) that can seal openings in formations to reducethe unwanted influx of fluids or the loss of drilling fluids topermeable formations. The mud cake forms when the drilling fluidcontains particles that are approximately the same size as or havediameters greater than about one third of the pore diameter (or thewidth of any openings such as induced fractures) in the formation beingdrilled. Drilling fluid additives can also increase the stability of thewellbore.

The drilling fluid must circulate in the wellbore (down the drill pipeand back up the annulus) in order to perform all of the desiredfunctions to allow the drilling process to continue. Therefore, drillingfluid should remain in the wellbore all the time, otherwise well controland caving in of the wellbore is immediate. Sometimes penetration ofundesirable formation conditions causes substantial to severe losses ofthe drilling fluid to the formation. The features responsible for suchlosses can be related to small to large fissures, or natural or inducedfractures in the formation; the losses may also be through highly porousrock.

Fluid loss is a common occurrence in drilling operations. Drillingfluids are designed to seal porous formations intentionally whiledrilling; this occurs as the result of suction of the fluid onto thepermeable surface (pressure greater in the well than in the formation)and the creation of a mud cake to seal a porous formation duringdrilling and for the purpose of wellbore stabilization. Some fluid willbe lost through the mud cake and fluid loss control additives arerequired.

However, the loss of fluids (the whole slurry) to the formation canreach an extent such that no mud cake can be created to secure thesurface and create an effective barrier. In extreme situations, when theborehole penetrates a fracture in the formation through which most ofthe drilling fluid may be lost, the rate of loss may exceed the rate ofreplacement. Drilling operations may have to be stopped until the lostcirculation zone is sealed and fluid loss to the fracture is reduced toan acceptable level. In the worst case, the consequences of this problemcan be loss of the well.

Curing lost circulation while drilling is the subject of manypublications, patents, and research for development of materials,reactions, and techniques is an ongoing process. Still the most greenerand desirable treatment is the physical blockage with specific solidmaterial. The benefit of the solids of any nature added to differenttypes of mud to cure losses is the possibility of separating them fromthe mud on the mud shakers and other equipments for general conditioningof mud for proper circulation in well. However, curing losseseffectively and quickly is still a matter of concern for many companiesand operators. The volumes of mud loss and the amounts of lostcirculation material used are both very great.

Over the years numerous techniques have been developed in order to cureor to reduce low to high lost circulation of mud to the wellbore. Underthese conditions, the normal procedure is to add fluid loss agents. Themechanism is to change the rheological properties of the drilling mud inorder to increase the resistance to flow of the fluid to formation. Thisalone may decrease the losses while drilling to an acceptable level.However, when it comes to intolerable losses it is now traditional toadd various bulk materials known as LCMs. Such prior art lostcirculation materials are selected from different groups of material inthe form of flake (or laminated), granular, and fibrous materials. Majormaterials mentioned are the cheap wastes of the other industries thatare used as LCM.

One of the traditional treatments to cure losses is the use of LostCirculation Materials (LCMs). As is well known to experts, conventionalLCMs are often not adequately efficient. One problem with conventionalLCMs is that they are generally waste products from other industriessuch a wood, paper, textile, agriculture, car, and different polymerproducts industry. These products are not engineered to be effective atblocking the loss zone. Not all materials are capable of building ablocking material with low permeability. The material may not only bedislodged and allow further fluid loss, but also may not pack in therequired way to be able to block losses. Regarding fibers, the situationis similar; not all fibers are capable of blocking fluid loss undergiven conditions, and selection and use of the wrong fiber can causegreat complications in mixing and pumping and ultimately with noblocking effect to cure losses.

The many LCMs that have been added to drilling fluids include cornstalks, wood shavings, flake cellophane, and chopped up paper (U.S. Pat.No. 2,610,149); rice hulls, shredded paper (U.S. Pat. No. 2,779,417);processed and shaped wet pulp residue with solid and fiber content withparticle sizes between 200-1000 micron with 70% inorganic filler(kaolinite clays and calcium carbonate) and 30% cellulous fiber withlength:diameter ratio of 2:1 (WO 93/18111); whole corncobs or the woodyring portion of corncobs (U.S. Pat. No. 4,247,403); sized coca beanshell material with a particle size distribution of 0.15 to 9.5 mm (2 to100 mesh) (U.S. Pat. No. 4,474,665); ground walnut shells, cellophane,and shredded wood (U.S. Pat. No. 4,579,668); a blend of rice fraction(40-90% by weight of drilling fluid with size finer than 65 mesh toabout 85 mesh, corn cobs, at least one ground wood fiber, ground nutshell, ground paper, and shredded cellophane (U.S. Pat. No. 6,323,158B1); oat hulls and one or more of ground corn cobs, cotton, citrus pulp,and ground cotton burrs (U.S. Pat. No. 5,004,553 and U.S. Pat. No.5,071,575); ground cotton burrs along with one or more of ground oathulls, ground corn cobs, cotton, ground citrus pulp, ground peanutshells, ground rice hulls, and ground nut shells (U.S. Pat. No.5,076,944); ground tannin-containing organic waste product includinggrape pumice, tomato pumice, yellow pine bark, yellow pine, wood bark,and the like (U.S. Pat. No. 6,399,545); and sugar cane fibers orbagasse, flax, straw, ground hemp, cellophane strips, ground redwoodfibers, ginned cotton fibers. Many of the natural products are at leastpartially fibrous in nature.

Organic and inorganic, natural and synthetic fibers have been used alongwith other particles and/or polymeric treatments. For example, in a curefor lost circulation, fibers were added to aqueous solutions ofpartially hydrolyzed polyacrylamide (U.S. Pat. No. 5,377,760). Inanother example, a high fluid loss spotting pill included a carrierfluid, a LCM containing acidizable mineral particulates, preferablycalcium carbonate (fine and coarse particles and fine flakes) along withacid-soluble mineral fibers, preferably fine fibers such as extrudedmineral wool having a diameter of 4 to 20 microns, preferably from about5 to 6 microns and a length of fiber about 200 microns, preferably 8 to25 microns (US 2003/0195120 and U.S. Pat. No. 6,790,812). In yet anotherexample, a loss circulation combination contained alkali metal silicateand water-insoluble particulate material as an integral component of awater-based drilling fluid system and a quantity of water-solubleactivating agent effective to reduce the pH of the water-based drillingfluid system low enough to cause precipitation of the silicate; thewater-insoluble particulate materials included cellulose fibers selectedfrom corn cobs, nut shells, seeds, pith, and lignin and had sizes fromabout 0.025 to about 2 mm (about 10 to about 500 mesh) (US 2007/0034410and U.S. Pat. No. 7,226,895). Coconut coir in the form of a mixture ofshort fibers, flakes, granular pieces, and powder from the coconut huskhas been used as an LCM in drilling fluid to prevent loss of drillingfluid into fractures in rock formations (US 2004/0129460). Finally,glass fibers or novoloid fibers in an amount of 1.43 to 17.12 kg/m³ (0.5to 6 lb/bbl) have been mixed with solid particles of less than 300microns (WO 2004/101704 and US 2007/0056730).

References listed above are incorporated herein by reference thereto.

Despite all these efforts, there is still a need for a preciselyengineered material that can quickly and efficiently reduce severe lostcirculation.

SUMMARY

A first embodiment is a composition for reducing lost circulation in awell; the composition contains a mixture of coarse, medium and optionalfine particles, and contains a blend of long fibers and/or short fibers.The fine particles preferably have an average particle size of fromabout 5 to about 15 microns. As an example, about 10 weight percent ofthe fine particles are smaller than about 1 micron and about 10 weightpercent of the fine particles are larger than about 30 microns. The fineparticles optionally have an average particle size of from about 5 toabout 10 microns. Preferably, about 10 weight percent of the mediumparticles are smaller than about 20 microns and about 10 weight percentof the fine particles are larger than about 150 microns. The mediumparticles, as an example, have an average particle size of from about 20to about 150 microns. Preferably, about 10 weight percent of the coarseparticles are smaller than about 5 microns and about 10 weight percentof the coarse particles are larger than about 1500 microns. As anexample, the coarse particles have an average particle size of fromabout 300 to about 1200 microns. The mixture of particles preferablycontains from about 0 to about 15 weight percent fine particles, about20 to about 40 weight percent medium particles, and about 40 to about 60weight percent coarse particles. The particles are selected fromalkaline earth carbonates, poly-paraphenyleneterephthalamide, mica,rubber, polyethylene, polypropylene, polystyrene,poly(styrene-butadiene), fly ash, silica, mica, alumina, glass, barite,ceramic, metals and metal oxides, starch and modified starch, hematite,ilmenite, microspheres, glass microspheres, magnesium oxide, gilsonite,sand, and mixtures of these materials. The fine particles may suitablybe calcium carbonate. The medium particles may suitably be rubber, mica,calcium carbonate poly-paraphenyleneterephthalamide, and mixtures ofthese materials. The coarse particles may suitably be mica, silica,calcium carbonate and mixtures of these materials. The mixture ofparticles may be about 8 to about 12 weight percent fine calciumcarbonate, about 30 to about 40 weight percentpoly-paraphenyleneterephthalamide, calcium carbonate, mica and mixturesthereof, and about 45 to about 60 weight percent coarse calciumcarbonate. Optionally, at least about 60 weight percent of the mixtureof particles is acid-soluble. In other embodiments, the long fibers arerigid and the short fibers are flexible. The weight ratio of long fibersto short fibers is preferably from about 1:4 to about 4:1. The lengthratio of long fibers to short fibers is preferably from about 1:1 toabout 3:1. Optionally, at least a portion of the fibers is acid-solubleand more preferably all the fibers are acid-soluble. Optionally, atleast a portion of the fibers is biodegradable. In other embodiments,the length of the long fibers is between about 8 and about 15 mm, thelong fibers are organic, the long fibers include water-insolublepolyvinyl alcohol, and/or the short fibers include water-solublepolyvinyl alcohol. By water soluble here it has to be understood solublein water at a temperature higher than the highest operating temperature.In yet other embodiments, the short fibers include a mixture of fibersof two different lengths, for example one type of fiber having anaverage length of from about 1 to about 2 mm and the second group offibers having an average length of from about 3 to about 8 mm.Optionally, the short fibers may include a mixture of two differentlengths of polyvinyl alcohol fibers; as examples, the two differentlengths are in a weight ratio of from about 90:1 to about 1:90, the twodifferent lengths are in a length ratio of from about 2 to about 6, orthe two different lengths are in a length ratio of from about 2.5 toabout 7. In other embodiments, the short fibers may include a mixture ofmultiple lengths of polyaramid fibers, the short fibers may includeinorganic fibers (for example made from calcium oxide and silica).Preferably, the fibers are made of polyvinyl alcohol, polyamide, aramid,para-aramid, polylactic acid, polyglycolic acid, metals, painted metals,polymer-coated metals, hollow metals, hollow painted metals, hollowcoated metals, polypropylene, polyethylene, polyester, polyamide,polyolefin, novoloid, phenol-aldehyde, nylon, rayon, extruded mineralwool, carbon, basalt, asbestos, and glass.

Another embodiment is a composition for reducing lost circulation in awell; the composition includes a mixture of coarse, medium and fineparticles, a blend of long fibers and short fibers, and a water-baseddrilling fluid. The mixture of particles is preferably added to thedrilling fluid at a concentration of from about 2.85 kg/m³ to about 130kg/m³, for example at a concentration of from about 14 kg/m³ to about 60kg/m³. The blend of fibers is preferably added to the drilling fluid ata concentration of from about 14 kg/m³ to about 42 kg/m³. Optionally, atleast a portion of the fibers is coated with a material that improvesthe dispersion of the fibers in the water-based drilling fluid. In oneexample, the long fibers include water-insoluble polyvinyl alcohol andthe short fibers include polyaramid fibers. Preferably, the mixture ofcoarse, medium and fine particles, and the blend of long fibers andshort fibers, is added at a total concentration of from about 2.85 kg/m³to about 142.5 kg/m³.

Yet another embodiment is a composition for reducing lost circulation ina well; the composition includes a mixture of coarse, medium and fineparticles, a blend of long fibers and short fibers, and an oil-baseddrilling fluid. The mixture of particles is preferably added to thedrilling fluid at a concentration of from about 2.85 kg/m³ to about 130kg/m³, for example at a concentration of from about 14 kg/m³ to about 60kg/m³. The blend of fibers is preferably added to the drilling fluid ata concentration of from about 14 kg/m³ to about 42 kg/m³. The longfibers may suitably include para-aramid. The short fibers may suitablyinclude para-aramid fibers and the long fibers may suitably includepolyaramid fibers. In various embodiments, the long fibers may be about8 to about 10 mm long, the long fibers may include polyaramid fibers,the long fibers may include polyvinyl alcohol fibers, the short fibersmay include a mixture of two different lengths, the short fibers mayinclude a mixture of multiple lengths of fibers, the short fibers mayinclude inorganic fibers, and both the long fibers and the short fibersmay include polyvinyl alcohol fibers. The mixture of coarse, medium andfine particles, and the blend of long fibers and short fibers, ispreferably added at a total concentration of from about 2.85 kg/m³ toabout 142.5 kg/m³.

Yet another embodiment is a composition for reducing lost circulation ina well; the composition includes a mixture of coarse, medium and fineparticles, and a blend of two different rigid fibers. The differentrigid fibers may include fibers of different length; the different rigidfibers may include fibers of different diameter. The different rigidfibers may suitably each include water-insoluble polyvinyl alcoholfibers.

Additional embodiments include methods of reducing lost circulation in awell. One such method involves adding a mixture of coarse, medium andoptional fine particles, and adding a blend of long fibers and shortfibers, to a drilling fluid. Another such method involves adding amixture of coarse, medium and optional fine particles to the drillingfluid, adding short fibers to the drilling fluid, and adding long fibersto the drilling fluid; the concentration of the long fibers is increasedduring the treatment. Yet another method involves adding a mixture ofcoarse, medium and optional fine particles to the drilling fluid, addingshort fibers to the drilling fluid, and adding long fibers to thedrilling fluid; the length of the long fibers is increased during thetreatment. Another method involves adding a mixture of coarse, mediumand optional fine particles, and adding a blend of two different rigidfibers to the drilling fluid; the two different rigid fibers preferablyhave diameters of from about 0.05 mm to about 0.2 mm and lengths of fromabout 8 to about 15 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the lost circulation testing apparatus.

FIG. 2 shows a cylinder containing a slit that mimics a fracture.

FIG. 3 illustrates the injection force required to deform mud cakesformed by a bentonite mud cake and bentonite mud cakes prepared withdifferent fibers.

DETAILED DESCRIPTION

It should be understood that throughout this specification, when aconcentration or amount range is described as being useful, or suitable,or the like, it is intended that any and every concentration or amountwithin the range, including the end points, is to be considered ashaving been stated. Furthermore, each numerical value should be readonce as modified by the term “about” (unless already expressly somodified) and then read again as not to be so modified unless otherwisestated in context. For example, “a range of from 1 to 10” is to be readas indicating each and every possible number along the continuum betweenabout 1 and about 10. In other words, when a certain range is expressed,even if only a few specific data points are explicitly identified orreferred to within the range, or even when no data points are referredto within the range, it is to be understood that the inventorappreciates and understands that any and all data points within therange are to be considered to have been specified, and that the inventorhas possession of the entire range and all points within the range.

We classify losses in four categories. Seepage losses happen very slowlyand can be confused with cutting removal at the surface. Seepage lossessometimes happen in the form of filtration to a highly permeableformation. They are characterized by losses of from about 0.16 to about1.6 m³/hr (about 1 to about 10 bbl/hr) of mud. If formation damage orstuck pipe is the primary concern, attempts should be made to curelosses before proceeding with drilling. Losses greater than seepagelosses but less than about 16 m³/hr (about 100 bbl/hr) are defined aspartial losses. In almost all circumstances when losses of this type areencountered, regaining full circulation is required. Losses of mud atrates greater than about 16 m³/hr (about 100 bbl/hr) are consideredsevere losses. As with partial losses, regaining full circulation isrequired. Traditional treatments for severe losses include spotting ofconventional LCM pills and moving to plugs if conventional treatmentsare not successful. The fourth category is total losses.

We have found a composition and method that is suitable for decreasingor eliminating seepage losses and partial losses, and severe lostcirculation in a drilling operation by blocking flow into permeableand/or fractured formations while drilling. The composition and methoddo not require a pH or temperature change. The composition and methodare particularly applicable to wells having partial losses or severelosses. In the latter case the composition and method provides, in afirst (primary) treatment, a temporary cure of severe losses. Forgreater assurance of a permanent and complete treatment, it isconvenient for a driller then to place a second treatment, such as acement plug. In that case the composition and method assure that thesecond treatment is effective. Additional benefits of the primaryplugging by the (first) treatment are lower total treatment cost, lessdamage to the formation that would have caused a decrease in thestability of the formation, and decreases in further problems that mayotherwise appear because of delays in treatment.

The present invention allows an immediate blockage of losses as soon astreatment places the new combination of fibers and particulate materialsin front of the zone byphysical mechanism. The invention helps to blockthe fracture at the entrance or pinch of the fracture in a way that canhandle the pressure in order to proceed with drilling well up toreaching intended target depth. Without wishing to be bound by anytheory, the inventors believe that the long fibers can flow in the mediabased on rigid or springy motion and that the short fibers are capableof snake like motion with less entanglement. The long fibers, enter thefracture and then align with the streamline of the mud or treatment flowand partition the fracture in order to trap the other particles tocreate a strong barrier against losses of mud to formation. The fiberselection plays an important role in this mechanism. Fibers should becompatible with each other and with the drilling fluid, and avoiddecrease in fluidity of the mud or treatment media, preferably even havea synergy with mud or treatment fluid. Blend of fibers preferably showssynergy with to distribute uniformly into the treatment fluid.

The composition comprises blends of (a) at least two different fibershaving specific and distinguishable individual characteristics alongwith (b) a specific blend of three different particle size ranges ofparticulate solids. One benefit of this treatment is that the solid andfiber content can be adjusted and evaluated with the mud carrier (forexample oil-based or water-based) used in the treatment.

A number of different systems has been found that can be used withwater-based and/or oil-based muds. The key to effective blockage is aproperly engineered blend of at least two fibers having very differentaspect ratios and flexibilities in combination with a specific blend ofthree sizes of solid particles. The presence of the fibers creates aneffective three dimensional heterogeneous network that can be blockedeffectively by solids having appropriate particle sizes to form a strongimpermeable mud cake. In addition, fine flexible fibers have astrengthening effect on the generated mud cake that further consolidatesthe entrance of a fracture or of the pore throats of permeable orunconsolidated formations during creation of an internal filter cake asthe result of leakage to the walls of the fracture. There is no need forthe mud or the mudcake to gel or to set (as would a cement).

The composition and method may be used with water-based and/or oil-baseddrilling fluids to eliminate problems associated from seepage losses tosevere losses, in particular so that other permanent solutions are notrequired. For use in cases near the upper limit of severe losses, thetreatment is recommended as a pre-treatment before a more consolidatedtreatment. This use as a pre-treatment decreases the total cost,decreases damage to the formation, and increases the chances of aneffective first placement of the secondary treatment (such as a cementplug or a reactive pill).

The fibers and solids are added to the drilling fluid (mud) in any orderand with any suitable equipment to form the treatment fluid. Typically,the fluid containing the fibers and solids is mixed before pumpingdownhole. The blend of fibers may be added and mixed and then themixture of solids added and mixed, or vice versa, or both fibers andsolids may be added before mixing. Typically the treatment fluid isweighted to approximately the same density as the drilling mud tominimize migration of the treatment fluid and mixing with the drillingmud. A weighting material may optionally be added to the fluid, thefibers, or the solids at any point. The treatment fluid may be added ina discrete amount, for example as a pill, or may be added until lostcirculation is satisfactorily reduced. The treatment fluid is spottedadjacent to the location of the lost circulation, if known, by methodsknown in the art.

The mixture of particles and blend of solids may be injected in severalstages in which the relative amounts of particles, long fibers, andshort fibers varies from stage to stage. Optionally, the length of thelong particles may also be less than optimal and then be increased tothe optimal length during the treatment. For example the concentrationof long fibers may put at lower-than-optimal concentration of fiber inthe first stage or stages of the treatment. A suitable low concentrationmay be determined by measuring the minimal effective blockingconcentration of the blocking material for a specific fracture size andthen using a concentration in the range of about 10 to about 90 percentof that minimal effective blocking concentration. For example, if theeffective concentration of the rigid fiber to cure a 2 mm fracture of isabout 8.56 kg/m³ (about 3 lb/bbl) then the low concentration may beselected from the range of about 0.856 to about 7.70 kg/m³ (about 0.3 to2.7 lb/bbl) of the rigid fiber. The low concentration selected should betested in the same equipment to validate the non-blocking effect of thetreatment. The treatment with low concentration of the rigid fiber isfollowed with a treatment with an effective concentration of fiberscapable of rapid blockage. As a result, treatment with the effectiveconcentration blocks the fracture at or near the wellbore and the lowconcentration rigid fiber plugs the fracture at a bottleneck deeper inthe fracture. In another case, the short fiber and rigid fiberconcentrations may be decreased by a certain percentage thatexperimentally may be determined based on the concentration of fibers inan effective fiber concentration for blocking a certain fracture size.The concentrations of the fibers in the first fluid may, for example, befrom about 5 to about 50 percent less than the effective minimalconcentration of fibers. The concentrations of each fiber type may bereduced by the same amount or by different amounts. In yet another case,in addition to a change in the fiber concentration, the amount of thesolid particles may also be decreased. The low concentration treatmentmay be designed in such a way that it blocks certain fracture sizeslower than the original fracture size. For example, the lowconcentration treatment may be designed to treat a 1 mm fracture and thefollowing treatment may be designed to treat a 4 mm fracture. Forexample, for a 1 mm fracture with using zero to a low concentration ofrigid fibers may be sufficient. When any of these strategies isfollowed, the treatment forms the blockage at least at two differentdepths in a fracture or in pores (one close to or at the wellbore andanother at a different depth in the fracture or pores away from thewellbore) rather than only at the wellbore or close to the wellbore.

A suitable blend of fibers and solids is selected for a given mud andgiven conditions, such as but not limited to the mud type, the bottomhole temperature and the extent of losses being experienced. Aneffective concentration of each component, or a range of effectiveconcentrations, may be identified by performing experiments such asthose described below. Similarly, suitable compositions of particles andfibers, and suitable particle size ranges and fiber lengths may beidentified by performing experiments such as those described below.

The composition and method may be used in any density mud; preferreddensities are from about 1.1 to about 2.0 kg/L (about 9.2 to about 17pounds per gallon (ppg)). The solids volume fraction may range fromabout 4 percent to about 50 percent. The amounts and type of particlesand fibers added may be chosen in a way that the added components do notchange the mud weights or solids volume fractions very much. As high aparticle concentration and/or as high a fiber concentration as can behandled by the on-site equipment may be used to help minimizepenetration of the treatment fluid into the fluid loss pathway.

Water-based muds are typically slurries of clay solids and polymers; theconcentrations and densities are adjusted to provide the specificproperties required for drilling, well protection, temperature control,and the other mud functions. Oil-based muds may contain diesel, polyalpha olefins, modified esters and ethers, mineral oils, otherhydrophobic materials, and mixtures of these. Oil-based muds may also beinvert emulsions of oil in which up to 50% water is dispersed in theoil; the oil is the external phase and water is the internal phase. Thecomposition may be added to drilling fluids in concentrations adjustedwith respect to the mud specifications.

The blend of solid particles preferably consists of coarse, medium, andfine particles. The blend may be used with water-based and oil-basedmuds. The coarse particles in the blend are preferably coarse carbonateshaving an average particle sizes above about 300 microns and less thanor equal to about 1200 microns. The carbonate is preferably calciumcarbonate. The medium size particles are preferablypoly-paraphenyleneterephthalamide powder, available from Teijin AramidCompany, Arnhem, The Netherlands. However, other materials may be usedfor these particles, for example any particles used in oilfield fluids,for example mica, calcium carbonate and cements. Calcium carbonateparticles having an average particle size of about 130 microns may beused. The medium particles may also be a mixture, for example a mixtureof calcium carbonate and from about 0 to about 95 percentpoly-paraphenyleneterephthalamide. The fine particles are preferablymicrofine carbonate particles having sizes below about 10 microns. Themain purpose of the fine particles is to facilitate metering andhandling of the blend; the fine particles may be left out if theequipment can handle the blend of medium and coarse particles. If thefine particles must be used and can invade small formation pores,non-damaging particles should be used. The ratio of thecoarse/medium/fine particles is preferably about 55/35/10 weightpercent. The coarse particles may vary from about 40 to about 60 percentof the mixture of particles; the medium particles may vary from about 30to about 40 percent of the mixture of particles; the fine particles mayvary from about 8 to about 15 weight percent of the mixture. Theparticles may be selected by one skilled in the art from any types ofparticles used in muds and are not limited to calcium carbonate andpoly-paraphenyleneterephthalamide. Coarse particle preferably have ahigh hardness to improve resistance to pressure. Such hardness istypically between 50 to 200 Vickers. Medium particle may have lowerhardness than the coarse particles.

Non-limiting examples of other suitable particles for use as componentsof the blend of solid particles include mica, rubber, polyethylene,polypropylene, polystyrene, poly(styrene-butadiene), fly ash, silica,mica, alumina, glass, barite, ceramic, metals and metal oxides, starchand modified starch, hematite, ilmenite, microspheres, glassmicrospheres, magnesium oxide, gilsonite, and sand. Mica is particularlysuitable because it provides substantial friction. Cement andmicrocement are not normally used as one or more of the particles, butmay be. If cement is present, it is less than about 50 weight percent,for example less than about 45 weight percent, of the weight of theparticles. The particles are not normally in flake or platelet form.Suitable wetting agents may be used to ensure that the materials areoil-wettable in oil-based muds or water-wettable in water-based muds.Laboratory tests should be performed to ensure compatibility with thedrilling fluid, that the fluid can transport the particles at thepumping rates used, and suitability for the size of the openings in thefluid loss pathways to be plugged.

The blend of fibers preferably contains fibers having different aspectratios and different flexibilities. The blend is most commonly a blendof two fibers, or a blend of three fibers but blends of more fibers maybe used. The fibers may optionally be a blend of different lengths offibers. Preferably, at least one fiber type is rigid and the rest of thefibers are flexible. Preferably, the rigid fibers are longer than theflexible fibers. The length of the longest particles is limited only bythe ability of the on-site equipment to accommodate and move the fibers,mix the fibers and fluid, and pump a fluid containing the fibers. Longrigid fibers may be effective alone at sufficient concentration forblocking some fluid loss pathways, but their efficacy is improved by theaddition of short fibers; these may also be rigid but are more effectiveif flexible. Note that we define a “flexible” fiber as having a Young'sModulus of less than about 20 GPa (kN/mm²) and a rigid fiber as having aYoung's Modulus of greater than about 20 GPa. Note that the fiberlengths specified are not intended to be precise; fibers as received, oras cut to length, inevitably are a mixture of lengths distributed aroundthe intended length.

In one preferred embodiment for water-based muds, the blend of fibers iscomposed of two types of fibers. One type of fiber is rigid and theother is flexible. The rigid fiber is preferably non-water-solublepolyvinyl alcohol. The second fiber may be selected from inorganic ororganic fibers. The second fiber may itself be a mixture of fibershaving the same diameter with different lengths or may have a singlefixed fiber length and preferably may be selected from aramid polymerssuitable for aqueous media and water-soluble polyvinyl alcohol polymerswhich are water-soluble at high temperature. In another preferredembodiment for water-based muds, the blend of fibers is composed ofthree fibers in which one fiber is an organic rigid long fiber and theother two fibers are inorganic and/or organic fibers (preferably bothorganic fibers) having different lengths. The fibers are generally cutto the appropriate lengths, by any method known in the art, from theas-received materials to provide the desired lengths. Note that fibersmay be described here as “water-soluble”, “non-water-soluble”, or“water-insoluble” because that is how they are described bymanufacturers and suppliers; water solubility or insolubility is notimportant to the Invention provided that the fibers do not dissolveunder bottomhole conditions before a mat or web of fibers has formed andtrapped the mixture of particles. In fact, fibers described by amanufacturer as insoluble may be insoluble at room temperature butdissolve within a few minutes at downhole temperatures; this may beadvantageous if the mudcake is later to be removed.

In another embodiment for water-based muds, the blend of fibers is ablend of two different rigid fibers, for example water-insolublepolyvinyl alcohol. The two different fibers may differ in length (forexample about 8 mm and about 12 mm or 6 mm and 12 mm), in diameter (forexample selected from about 40, about 100 and about 200 microns), or inrigidity (tensile strength).

In one embodiment for water-based muds, preferred fibers areacid-soluble fibers that have good performance in highly alkalineenvironments. Formations having even severe losses can thus be treatedwith water-based systems. We define acid-soluble as soluble in anaqueous acid solution commonly injected into wells in the oilfield, forexample formic acid, acetic acid, citric acid, hydrochloric acid (forexample 3 percent or 25 percent), or mixtures of these. The fibers aretherefore subsequently removable with HCl, for example 15% HCl, if it isdesirable to remove the mud cake. When some or all of the particulatesolids includes carbonate solids, they too are acidizable. In general,the higher the content of particulate solids, the more effective is themud cake at blocking fractures and high permeability or unconsolidatedregions.

One preferable long rigid organic fiber for water-based muds is anon-water-soluble polyvinyl alcohol fiber. The length of these fibers ispreferably in the range of about 10 to about 15 mm, more preferablyabout 12 mm. The diameter of these fibers is preferably about 0.04 mm to0.2 mm, the tensile strength preferably about 1000 N/mm² (1.0 GPa), andthe elongation preferably about 8 KN/mm² Preferred polyvinyl alcoholfibers have a thermal decomposition temperature typical of bottom holeconditions, for example around 220° C.; at this temperature thepolyvinyl alcohol polymer reacts readily with water. Such fibers may beobtained from Kuraray Inc., Osaka, Japan.

In one preferred embodiment for water-based muds the flexible fiber maybe an inorganic mineral fiber largely composed of CaO and SiO₂, and alsotypically containing significant amounts of Al₂O₃, MgO, and Fe₂O₃,commonly coated with a monomolecular film of specially formulatedsurfactant to ease the separation of fibers when they are added to water(improve the dispersion). Such a fiber is MAGMA™ fiber available fromLost Circulation Specialists, Inc., Casper, Wyo., USA. This fiber isacid-soluble and thermally stable at temperatures up to 1,800 degrees.The specific gravity of those fibers is 2.6 with no tendency to float.MAGMA™ fiber is an inert non-damaging material towards the environmentwith an LC-50 of one million. MAGMA™ fiber is available in a “fine” formhaving a length of from about 0.1 to about 4 mm and a “regular” formhaving a length of from about 4 to about 20 mm with an average length ofabout 10 to about 16 mm. The fiber diameters of both grades of MAGMA™fiber ranges from about 5 to about 15 microns with an average diameterof about 7 to about 10 microns. MAGMA™ fibers are obtained as mixturesof multiple sizes. Fiber for use in the Invention may optionally be cutfrom either form as appropriate.

In another preferred embodiment for water-based muds the flexible fibermay be a biodegradable organic fiber, for example polyvinyl alcoholfibers which are soluble in water at high temperatures. Polyvinylalcohol is available in a series of compositions that are water-solubleat specific temperatures, and soluble in 15% HCl at high temperatures.In highly alkaline media, dissolution of fibers occurs at highertemperatures (approximately 10° C. above the dissolution temperature inneutral water). A preferred mixture of flexible fibers is a blend of twodifferent lengths of fibers made with the same polymer, blended so thatthey have a length ratio of from about 2 to about 6. Such fibers may beblended at a ratio of about 10:90 wt/wt % to about 90:10 wt/wt % of thefibers of different lengths.

In various preferred embodiments for water-based muds, the flexiblefiber is a short cut organic wet pulp type polyaramid material,available as “wet pulp” containing about 4 to 7 percent water or as“yarn”, each having a dispersant finish, and suitable for use in aqueousmedia, for example TWARON™ polyaramid obtained from Teijin, having abroad range of short fiber lengths; the blend of fiber contains twofibers in which the rigid fiber is a non-water-soluble rigid polyvinylalcohol fiber and the second fiber is an inorganic fiber, for exampleMAGMA™; the blend of fibers contains two fibers in which one fiber is anon-water-soluble rigid polyvinyl alcohol fiber and one fiber is awater-soluble polyvinyl alcohol fiber that is soluble at hightemperature and the length ratio of the rigid to the flexible fibers isfrom about 1 to about 3; the blend of fibers is a blend of three fibersin which one is a rigid fiber that is non-water-soluble polyvinylalcohol fiber and the other two fibers are short cut polyvinyl alcoholfibers as previously described having different lengths, with lengthratios of the flexible fibers ranging between about 2.5 and about 7; thepreferred length of the rigid fiber, for example non-water-solublepolyvinyl alcohol fibers, is about 12 mm and the preferred length of theflexible fiber, for example water-soluble polyvinyl alcohol, is betweenabout 1.5 mm and about 6; and the blend of fibers contains two fibers inwhich the rigid fiber is non-water-soluble polyvinyl alcohol fiber andthe other organic fiber is TWARON™ polyaramid “wet pulp” suitable for anaqueous environment.

In one embodiment particularly suited to oil-based muds, the blend offiber is composed of two fibers. One fiber is rigid and the other isflexible. The rigid fiber is preferably an organic polymer selected fromnon-water-soluble polyvinyl alcohol and polyaramid. The second fiber isselected from inorganic fibers, preferably a mixture of fine diameterfibers having different short lengths, or organic fibers; the organicfibers are preferably selected from aramid polymer, and water-solublepolyvinyl alcohol polymers that are water-soluble at high temperature.

In another embodiment particularly suited to oil-based muds, the blendof fibers includes three fibers in which one fiber is a long rigidorganic fiber and the other two fibers are a different organic fiber andhave differing shorter lengths. Preferred fibers are acid-soluble exceptfor polyaramid fibers. However, those fibers have very good performancein highly alkaline environments. The blocking materials preferably areat least partially removable by 15% HCl.

The preferred mixture of solid particles for use with oil-based muds isthe same mixture as that used for water-based muds. Preferred fibers foruse with oil-based muds include organic and inorganic fibers. Theorganic fibers may be water-soluble at high temperature or may benon-water-soluble fibers or oil-dispersible fibers. Preferred blends offibers contain one long rigid fiber with the rest being a shorter moreflexible fiber that may be a mixture of lengths.

One preferred long rigid organic fiber for use with oil-based muds isthe same as the long rigid organic fiber used with water-based muds, forexample non-water-soluble polyvinyl alcohol fibers as described above.Another preferred long rigid organic fiber for use with oil-based mudsis polyaramid polymer fibers such as TECHNORA™ Para-Aramid fibersavailable from Teijin Aramid Company, Arnhem, The Netherlands. Suchfibers are3,4′-diaminodiphenylether-para-phenylenediamine-terephthaloyldichloridecopolymers with 3,4′-oxydianilino-para-phenyelenediamine-terephthalicacid; they are finished with about 2% of an antistaticadditive/lubricant, typically having a decitex of 1.2 to 1.7, a densityof 1.39 gr/cm³, and a length of 10 to 15 mm. The fibers are about twiceas strong as fiber glass and nylon fibers of the same weight. This fiberdecomposes at about 500° C., can be used at 200° C. for very long times,and maintains half of its room temperature tensile strength at 250° C.The TECHNORA™ Para-Aramid fibers are typically cut to about 15 mmlengths before use.

One preferred short flexible inorganic fiber for use with oil-based mudsis the MAGMA™ fiber, available from Lost Circulation Specialists, Inc.,Casper, Wyo., USA, describe above. A preferred short flexible organicfiber for use with oil-based muds is the polyvinyl alcohol fiber whichis soluble in water at high temperatures; this was also described inmore detail above. Another preferred short flexible organic fiber foruse with oil-based muds is short cut polyaramid fibers having a lengthin the range of from about 0.5 to about 8 mm and a filament diameter ofabout 12 microns. As an example, such fibers, in the form of choppedyarn bundles of approximately 1 mm diameter, are available commerciallyfrom Teijin as TWARON 1092™ (mean fiber length about 1.4 mm but a fairlybroad distribution) and TWARON 1094™ (bimodal mean fiber lengths ofabout 0.5 and 1.4 mm but fairly broad distributions). The fibers have adensity of 1.44 g/cm³.

In various embodiments suitable for use with oil-based muds, the blendof fiber is composed of two fibers in which one is a rigid fiber that ispreferably a non-water-soluble rigid polyvinyl alcohol fiber such asthose described above available from Kuraray and the second fiber is aninorganic fiber such as the MAGMA™ also described above; the blend offiber is composed of two fibers in which one fiber is non-water-solublerigid polyvinyl alcohol fiber as described above and the other fiber isa water-soluble polyvinyl alcohol fiber that is soluble at hightemperature (also described above) and the length ratio of the rigid tothe flexible fibers is about 1 to about 3; the blend of fibers includesthree fibers in which the rigid fiber is non-water-soluble polyvinylalcohol fiber as described above and the other two fibers are short cutpolyvinyl alcohol fibers as described above in which the short cutfibers have two different lengths with a ratio ranging from about 2.5 toabout 7; the preferred lengths of fibers are about 1.5 mm and 6 mm forpolyvinyl alcohol water-soluble fibers and 12 mm for polyvinyl alcoholnon-water-soluble fibers; the blend of fibers contains two fibers inwhich the rigid fiber is polyaramid polymer fiber such as the TECHNORA™Para-Aramid fibers mentioned above and the flexible fiber is short cutpolyaramid fiber such as TWARON 1092™ or TWARON 1094™

For either water-based or oil-based muds, other fibers may be used.Laboratory tests should be performed to ensure compatibility with thedrilling fluid, that the fluid can transport the fibers at the pumpingrates used, and suitability for the size of the openings in the fluidloss pathways to be plugged. Non-limiting examples of other suitablefibers include metals, painted metals, polymer-coated metals, hollowmetals, hollow painted metals, hollow coated metals, polypropylene,polyethylene, polyester, polyamide, polylactic acid, polyglycolic acid,polyolefin, novoloid such as phenol-aldehyde, nylon, rayon, extrudedmineral wool as described in U.S. Pat. No. 6,790,812, carbon, basalt,asbestos, and glass. Metallic fibers are particularly suitable at hightemperatures. Suitable wetting agents may be used to ensure that thematerials are oil-wettable in oil-based muds or water-wettable inwater-based muds.

In another embodiment for water based and oil based muds, in particularat high temperatures, the rigid fibers may be made of metal that may becoated or non-coated, and the flexible fibers may be fine extrudedflexible fibers or, for example, MAGMA™ fiber. The rigid metallic fibersmay optionally be hollow, ribbon-shaped or cylindrical, and the shortflexible fibers may optionally be fiber mesh.

Embodiments may be further understood from the following examples.

Examples of Compositions Particularly Suited for Water-Based Muds

These experiments were performed in a low density water-based mud. Theblend of solid particles consisted of coarse calcium carbonate having aparticle size of from about 300 to about 1200 microns, medium particlesof TWARON™ 5001 polyaramid polymer powder made ofpoly-paraphenyleneterephthalamide having a bulk density of about 325kg/m3, and fine calcium carbonate having an average particle size ofabout 10 microns (MIKHART™ 10 available from, M-I Swaco, Houston, Tex.USA. The coarse/medium/fine ratio was always 55/35/10 weight percent inthese experiments.

All the tests were performed in a modified lost circulation cell, FIG.1, equipped with one of two modified slits through a cylinderapproximately 50 mm high having a 1 mm or 2 mm opening, as shown in FIG.2. The experimental set up consists essentially of a high-pressurehigh-temperature fluid loss cell [2] that is equipped with the cylinder[6] at the bottom. Pressure was applied from the top of the cell ontofluid [4] placed in the cell (as in traditional fluid loss experiments).When the cell was ready, a primary pressure of 10 bars was applied andthe valve was opened to simulate the differential pressure at two endsof a fracture. The pressure was held for at least 30 minutes. The lossof the mud was monitored by a balance that was connected to a computer.The mud weight vs. time was recorded and plotted. Water-based muds wereprepared, allowed to sit for two hours, and then were sheared beforeaddition of any particles or fibers for 10 minutes in a Hamilton Beachmixer (traditionally used for mud preparation for less than 500 mL).Note that the mud was fresh. The muds were not aged because it wasassumed that in the field, when mud losses occur, a treatment fluid willbe prepared at the site and probably not aged for very long so that itcan be used to cure the losses quickly. In any case, the particle sizesof mud additives are not suitable for effective blockage. The mud wasthen inspected to assure proper blending and proper homogeneity of themud. Solids were then added with a Heidolph mixer. The rpm was adjustedso that a vortex was observed; 500 rpm was usually suitable. In thefirst step, typically 10 grams of solid blend was added to 300 mL overthe course of 2 minutes to the mud samples. (Different volumes wereoccasionally used because of the limitations of certain muds.) Thesolids were then mixed with the mud for 10 minutes. In the second step,fibers were added to the blend over the course of 2 minutes and left tobe blended for an additional 10 minutes for complete dispersion.Checking of the homogeneity of the entire blend is mandatory.

The blend of solid particles was 55:35:10 weight percent of coarsecalcium carbonate having an average particle size of 700 microns,poly-paraphenylene-terephthalamide, and fine calcium carbonate having anaverage particle size of 10 microns. This blend of solid particles wasadded to all the muds used here in an amount of 36.23 kg/m³ (12.7 poundsper barrel) of water-based mud.

Example 1

Tests of Examples 1 through 5 were performed with 1.14 kg/L (9.50 ppg)water-based mud. The water-based mud was composed of 3.4 g/L FLOVIS™xanthan viscosifier available from M-I Swaco, 10.86 g/L DUALFLO™modified starch available from M-I Swaco, 3.15 g/L magnesium oxide, 31g/L KCl, 40 g/L HYMOD PRIMA™ ball clay available from Imerys Minerals,Par, England, and was weighted with Barite.

To the water-based mud (containing the solid particles), was added 5.70kg/m³ (2 lb/bbl) polyvinyl alcohol fibers soluble in water at hightemperature, half of which had a length of about 1.5 mm and half ofwhich had a length of about 6 mm (KURALON™ WN8 fiber mixture availablefrom Kuraray, Osaka, Japan) and 5.70 kg/m³ (2 lb/bbl) non-water-solublepolyvinyl alcohol having a length of about 12 mm (KURALON™ RF400 fiberavailable from Kuraray, Osaka, Japan). A 343 g portion of themud/solids/fibers system was poured into the lost circulation cell, andunder a 6.89 bar (100 psi) differential pressure, the slurry quicklyblocked the modified 2 mm slit. As was typical for successful blockage,the bulk of the fluid loss occurred when the pressure was first applied;within seconds the effluent changed from the appearance of mud to theappearance of water. The amount of mud lost was 76 g.

Example 2

To the water-based mud above (containing the solid particles), was added2.85 kg/m³ (1.0 lb/bbl) polyvinyl alcohol fibers soluble in water athigh temperature, half of which had a length of about 1.5 mm and half ofwhich had a length of about 6 mm (KURALON™ WN8 fiber mixture availablefrom Kuraray, Osaka, Japan), and 8.55 kg/m³ (3 lb/bbl) non-water-solublepolyvinyl alcohol having a length of about 12 mm (KURALON™ RF400 fiberavailable from Kuraray, Osaka, Japan). A 343 g portion of themud/solids/fibers system was poured into the lost circulation cell, andunder a 6.89 bar (100 psi) differential pressure, the slurry blocked themodified 1 mm slit. The amount of mud lost was 11 g.

Example 3

To the water-based mud above (containing the solid particles), was added8.55 kg/m³ (3 lb/bbl) long flexible “regular” MAGMA™ fiber (availablefrom Lost Circulation Specialists, Inc., Casper, Wyo., USA) (describedabove) and 8.55 kg/m³ (3 lb/bbl) non-water-soluble polyvinyl alcoholhaving a length of about 12 mm (KURALON™ RF400 fiber available fromKuraray, Osaka, Japan). A 320 g portion of the mud/solids/fibers systemwas poured into the lost circulation cell, and under a 6.89 bar (100psi) differential pressure, the slurry blocked the modified 1 mm slitblockage after 9 g of mud loss.

Example 4

To the water-based mud above (containing the solid particles), was added8.55 kg/m³ (3 lb/bbl) long flexible “regular” MAGMA™ fiber (availablefrom Lost Circulation Specialists, Inc., Casper, Wyo., USA) (describedabove), and 8.55 kg/m³ (3 lb/bbl) non-water-soluble polyvinyl alcoholhaving a length of about 12 mm (KURALON™ RF400 fiber available fromKuraray, Osaka, Japan). A 322 g portion of the mud/solids/fibers systemwas poured into the lost circulation cell, and under a 6.89 bar (100psi) differential pressure, the slurry blocked the modified 2 mm slitblockage after 24 g of mud loss.

Example 5

To the water-based mud above (containing the solid particles), was added2.85 kg/m³ (1.0 lb/bbl) polyvinyl alcohol fibers soluble in water athigh temperature, half of which had a length of about 1.5 mm and half ofwhich had a length of about 6 mm (KURALON™ WN8 fiber mixture availablefrom Kuraray, Osaka, Japan) and 8.55 kg/m³ (3 lb/bbl) non-water-solublepolyvinyl alcohol having a length of about 12 mm (KURALON™ RF400 fiberavailable from Kuraray, Osaka, Japan). A 313 g portion of themud/solids/fibers system was poured into the lost circulation cell, andunder a 6.89 bar (100 psi) differential pressure, the slurry blocked themodified 2 mm slit. The amount of mud lost was 53 g.

Example 6

The mud used to prepare the mud cake was a bentonite mud having adensity of 1.65 kg/L (13.8 ppg). It contained 412.5 g/L of bentonite gel(which in turn was 85.6 kg/m³ (30 lb/bbl) bentonite in water) and 830g/L of barite in water. Two mud cakes were made with this mud and withthe addition of one or the other of two types of flexible fiber to thismud. The fibers used with this mud for preparation of mud cakes wereglass fibers (about 20 microns in diameter) cut in the laboratory intoapproximately 1-2 mm lengths, and “regular” MAGMA™ fiber. The amounts ofthe MAGMA™ fiber and glass fiber added to the mud in two experimentswere 14.98 and 14.27 kg/m³ (5.25 and 5.00 lb/bbl), respectively.

The mud cakes were prepared in a cell under a pressure of 10 bar (alittle above the 6.89 bar (100 psi) pressure used in the API Fluid Losstest for mud (ARTC-LP-070)). The prepared mud cakes were tested by aninjection test to evaluate the mud cakes' resistance to deformation andtheir injectability into a small opening. These properties were measuredby testing the mud cakes' resistance to deformation as the result offorce exerted on the mud cakes to deform and inject them into a narrowtip. An increase in the energy required to deform a mud cake is relatedto an increase in the strength of the mud cakes' structure. The samplingequipment for the injection tests was a 5 mL syringe having a metallicpiston; the tip of the syringe was removed and replaced with a threadedtip having an internal hole with nominal dimensions of 4.15 mm diameterand 33.30 mm length. With the piston in the syringe but the threaded tipnot on, the open end of the syringe was gently pushed into the mud cakeso that mud cake entered the syringe and pushed up the piston. Thethreaded tip was then placed on the syringe after the mud cake was inplace. The syringe was then mounted in a frame for stability andpressure was slowly exerted on the piston with a device that couldmeasure the force as a function of time and distance. The forcecompressed the sample and the mud flowed into the threaded tip; fillingof the tip was assured by loosely placing a small screw having a hole inthe middle (approximately 2.1 mm) on the end of the threaded tip toproduce an additional resistance for the mud cake, causing the mud caketo occupy the entire volume of the tip. After ensuring proper filling ofthe tip, the screw was removed and the process was continued. As thepiston moved downward and approached the bottom of the syringe, thepressure in the tip was increased steadily up to the measurement limitof 800 N.

FIG. 3 shows the injection profile and the force required to inject mudcake samples, made from the bentonite mud with and without glass orMAGMA™ fiber, into a narrow tip are shown. Mud cake made from thebentonite mud was injected much more easily into the tip than the mudcakes that contained fibers. The mud cake that contained “regular”MAGMA™ fiber required more force to be injected into the tip incomparison to the mud cake containing the glass fibers. The presence ofdifferent lengths of the MAGMA™ fibers (in the as-received material)with their small diameters was responsible for the creation of astronger three-dimensional structure and greater heterogeneity in themud cake compared to the mud cake made with the glass fibers. All of theinjection profiles showed plateaus. Not intending to be limited bytheory, it is believed that an approximately horizontal plateauindicates either shearing of successive layers of the sample or acombination of simultaneous shearing, extrusion and adhesion. Lessreproducibility was observed with the mud cakes containing the MAGMA™fiber because of the very random distribution of fibers and theirorientation. The glass fibers had a more uniform length. Longer lengthsof glass fibers (10 mm) were tried but were not injectable through thesmall nozzle being used; therefore the resistance increased with thelength of the fiber.

Examples of Compositions Particularly Suited for Oil-Based Muds

The following experiments were performed with a synthetic oil-based mud.All tests with the oil-based mud used the same blend of solidparticulates as used in the experiments with the water-based mud and thesame modified lost circulation cell equipped with the 1 mm modifiedslit.

Tests were performed using a 1.51 kg/L (12.5 ppg) oil-based mud. The mudis an ester-based synthetic fluid (base fluid ECOGREEN™ B, esters ofnatural fatty acids such as palm, coconut and fish oils, and ethanol)and is made using 15 l/m³ ECOGREEN™ P surfactant that is the primaryemulsifier (available from M-I Swaco) that forms a tight, stablebrine-in-ester fluid, 12 l/m³ ECOGREEN™ S multi-functional aliphaticester that performs as a secondary emulsifier (available from M-ISwaco), 10 kg/m³, ECOGREEN™ F fluid loss control agent (available fromM-I Swaco), 4 kg/m³ VG PLUS™ organophilic bentonite clay available fromM-I Swaco, 60 kg/m³ CaCl₂, 20 kg/m³ lime, and barite to mud weight.

Example 7

To the oil-based mud just described was added 2.85 kg/m³ (1.0 lb/bbl)TECHNORA™ Para-Aramid rigid fiber cut to about 1 mm length and 2.85kg/m³ (1.0 lb/bbl) flexible TWARON™ polyaramid fiber, cut toapproximately 8-10 mm length. A 475 g portion of the mud/solids/fiberssystem was poured into the lost circulation cell, and under a 6.89 bar(100 psi) differential pressure, the slurry blocked the modified 1 mmslit after 257 g of mud loss. Note that in this example, the flexiblefibers were the long fibers and the rigid fibers were the short fibers.The treatment was not as effective as were the treatments in the otherexamples, in which the longer fibers ere rigid and the shorter fiberswere flexible, but it did work.

Example 8

To the oil-based mud just described was added 2.85 kg/m³ (1.0 lb/bbl)flexible polyvinyl alcohol water-soluble at high temperature (previouslydescribed) cut to about 1.5 mm length, 2.85 kg/m³ (1.0 lb/bbl) of thesame polyvinyl alcohol water-soluble at high temperature cut to about 6mm length, and 5.70 kg/m³ (2 lb/bbl) non-water-soluble rigid polyvinylalcohol (previously described) cut to about 12 mm. A 500 g portion ofthe mud/solids/fibers system was poured into the lost circulation cell,and under a 6.89 bar (100 psi) differential pressure, the slurry blockedthe modified 1 mm slit; the mud loss was 7 g.

Example 9

To the oil-based mud just described was added 8.55 kg/m³ (3.0 lb/bbl)MAGMA™ fiber (described above) and 8.55 kg/m³ (3.0 lb/bbl)non-water-soluble polyvinyl alcohol, as described above, cut to 12 mmlength. A 482 g portion of the mud/solids/fibers system was poured intothe lost circulation cell, and under a 6.89 bar (100 psi) differentialpressure, the slurry blocked the modified 1 mm slit the mud loss was 14g.

The Invention is applicable to wells of any orientation. The Inventionmay be used for wells for production of hydrocarbons or other fluids,such as water or carbon dioxide, or, for example, for injection orstorage wells.

1. A composition for reducing lost circulation in a well comprising amixture of coarse particles having an average particle size of from 300to 1200 μm, medium particles having an average particle size of from 20to 150 μm and optionally fine particles having an average particle sizeof from 5 to 15 μm, and a blend of long fibers having an average lengthof from 8 to 15 mm and short fibers having an average length of from 1to 8 mm.
 2. The composition of claim 1 wherein the mixture of particlescomprises from about 0 to about 15 weight percent fine particles, about20 to about 40 weight percent medium particles, and about 40 to about 60weight percent coarse particles.
 3. The composition of claim 1 whereinthe particles are selected from the group consisting of alkaline earthcarbonates, poly-paraphenyleneterephthalamide, mica, rubber,polyethylene, polypropylene, polystyrene, poly(styrene-butadiene), flyash, silica, mica, alumina, glass, barite, ceramic, metals and metaloxides, starch and modified starch, hematite, ilmenite, microspheres,glass microspheres, magnesium oxide, gilsonite, sand, and mixturesthereof.
 4. The composition of claim 1 wherein the mixture of particlescomprises about 8 to about 12 weight percent fine calcium carbonate,about 30 to about 40 weight percent poly-paraphenyleneterephthalamide,mica or calcium carbonate, and about 45 to about 60 weight percentcoarse calcium carbonate.
 5. The composition of claim 1 wherein at leastabout 60 weight percent of the mixture of particles is acid-soluble. 6.The composition of claim 1 wherein the fibers are selected from thegroup consisting of polyvinyl alcohol, polyamide, aramid, para-aramid,polylactic acid, polyglycolic acid, metals, painted metals,polymer-coated metals, hollow metals, hollow painted metals, hollowcoated metals, polypropylene, polyethylene, polyester, polyamide,polyolefin, novoloid, phenol-aldehyde, nylon, rayon, extruded mineralwool, carbon, basalt, asbestos, and glass.
 7. The composition of claim 1wherein the weight ratio of long fibers to short fibers is from about1:4 to about 4:1.
 8. The composition of claim 7 wherein the length ratioof long fibers to short fibers is from about 1 to about
 3. 9. Thecomposition of claim 1 wherein at least a portion of the fibers isacid-soluble.
 10. The composition of any of claim 1 wherein the longfibers comprise water-insoluble polyvinyl alcohol and the short fiberscomprise water-soluble polyvinyl alcohol.
 11. The composition of claim 1wherein the short fibers comprise a mixture of fibers of two differentlengths.
 12. The composition of claim 11 wherein the short fiberscomprise a mixture of two different lengths of polyvinyl alcohol fibers.13. The composition of claim 11 wherein the short fibers comprise amixture of multiple lengths of polyaramid fibers.
 14. A composition forreducing lost circulation in a well comprising a mixture of coarseparticles having an average particles size of from 300 to 1200 μm,medium particles having an average particles size of from 20 to 150 μmand optionally fine particles having an average particles size of from 5to 15 μm, and a blend of long fibers having an average length of from 8to 15 mm and short fibers having an average length of from 1 to 8 mm.,and a drilling fluid.
 15. The composition of claim 14 wherein themixture of particles is added to the drilling fluid at a concentrationof from about 2.85 kg/m³ to about 130 kg/m³.
 16. The composition ofclaim 14 wherein the blend of fibers is added to the drilling fluid at aconcentration of from about 14 kg/m³ to about 42 kg/m³.
 17. Thecomposition of claim 14 wherein at least a portion of the fibers iscoated with a material that improves the dispersion of the fibers in thedrilling fluid.
 18. The composition of claim 14 wherein the drillingfluid is water-based, the long fibers comprise water-insoluble polyvinylalcohol and the short fibers comprise polyaramid fibers, water solublepolyvinyl alcohol fibers, or magma fiber.
 19. The composition of claim14 wherein the drilling fluid is oil-based and the long fibers comprisewater-insoluble polyvinyl alcohol or para-aramid.
 20. The composition ofclaim 14 wherein the drilling fluid is oil-based, the short fiberscomprise para-aramid fibers and the long fibers comprise polyaramidfibers.
 21. The composition of claim 14 wherein the drilling fluid isoil-based, the long fibers comprise non-water soluble polyvinyl alcoholfiber or magma fiber.
 22. The composition of claim 14 wherein the longfibers comprise polyaramid fibers or polyvinyl alcohol fibers.
 23. Thecomposition of claim 14 wherein the short fibers comprise a mixture oftwo different lengths or a mixture of multiple lengths of fibers. 24.The composition of claim 14 wherein both the long fibers and the shortfibers comprise polyvinyl alcohol fibers.
 25. A composition for reducinglost circulation in a well comprising a mixture of coarse particleshaving an average particle size of from 300 to 1200 μm, medium particleshaving an average particle size of from 20 to 150 μm and fine particleshaving an average particle size of from 5 to 15 μm, and a blend offibers having a Young's Modulus of greater than about 20 GPa.
 26. Thecomposition of claim 25 wherein the different rigid fibers each comprisewater-insoluble polyvinyl alcohol.
 27. A method of reducing lostcirculation in a well comprising adding a mixture coarse particleshaving an average particles size of from 300 to 1200 μm, mediumparticles having an average particles size of from 20 to 150 μm andoptionally fine particles having an average particles size of from 5 to15 μm and adding a blend of long fibers having an average length of from8 to 15 mm and short fibers having an average length of from 1 to 8 mm,to a drilling fluid and injecting the fluid into the well.